I recently ordered an SM846 system off ebay, and when I got it, it came with mismatched rails (Rev B inners and Rev A outers), so I had to try and figure out which rails were which as well (and then convince the seller to send me a matching set of inner rails, which they ultimately did). My Rev A outer rails have no markings on them what-so-ever to indicate what Rev or even what Part Number they are. I did some google image searching and I guess that Rev B rails are typically marked on the outside edge of the extension slide, near the front. There should be a sticker there saying "Rev B". The inner rails are correspondingly marked on the inside of the rail near the back, so that when you're about to push the inner and outer rails together, both stickers should be plainly visible. If you see two Rev B stickers, you've got Rev B rails. If you see no stickers, you've got Rev A rails (or Rev B rails that someone peeled the sticker off).
I think the main difference between the rails that makes them incompatible is the height. I measured mine and got:
Rev A Inner - ~40.6mm tall
Rev B Inner - ~39.24 mm tall
Rev A Outer - ~39.4mm to the inside of the ball bearings.
The hole pattern on the inner rails differ slightly from Rev A to Rev B, but I don't think it affects the function or how it attaches to the chassis.
This is for the 2U/3U MCP-290-00053-0N "yellow label" 26.5" rail kit.